Branch Coverage

What it is

Branch coverage (also called decision coverage) measures whether every branch from every decision point in the code has been taken at least once. A decision point is any point where execution can split: if/else, switch, while, for, ternary operators.

For every decision, there are at least two branches: the path taken when the condition is true, and the path taken when it’s false. 100% branch coverage requires both to be tested.

Worked example

function getDiscount(user, total) {
  let discount = 0;
  if (user.isMember) {          // Decision 1: TRUE branch / FALSE branch
    discount = 0.10;
    if (total > 100) {          // Decision 2: TRUE branch / FALSE branch
      discount = 0.15;
    }
  }
  return discount;
}

Three tests achieve 100% branch coverage — and in doing so, they also achieve 100% statement coverage. Branch coverage always subsumes statement coverage.

vs statement coverage

100% statement coverage is achievable with a single test (member, total = 150). That test never triggers the non-member path or the below-$100 path. Branch coverage forces you to test those paths too.

Coverage targets by context

• Safety-critical / financial code: 100% branch coverage, plus MC/DC (Modified Condition/Decision Coverage)
• Core business logic: 100% branch coverage
• Standard application code: 80%+ branch coverage is a common industry target
• Generated or boilerplate code: exclude from measurement

ISTQB mapping

function calculateReward(user, purchaseAmount) {
  let points = 0;
  if (user.isLoyaltyMember) {          // D1
    points = purchaseAmount * 2;
    if (purchaseAmount >= 100) {       // D2
      points = points * 1.5;            // bonus multiplier
    }
  }
  return points;
}